Integrated circuits often include devices formed within a semiconductor substrate and multiple levels of interconnect layers, which include conductive features separated by dielectric material. Over the years, the performance of microelectronic integrated circuits, with respect to speed and power consumption, has been improved by reducing the size of the integrated circuit, especially devices formed within the semiconductor substrate.
As the spacing between interconnect conductive features within the integrated circuit continuously decreases, the integrated circuit becomes increasingly susceptible to capacitance coupling between two or more conductive features. In particular, the integrated circuit becomes increasingly susceptible to crosstalk due to increased capacitive coupling between the conductive features and to signal propagation caused by interconnect resistailnce and capacitance ("RC delays").
The capacitance between two or more interconnect conductive features is proportional to the dielectric constant, k, of material separating the features and inversely proportional to the distance between the conductive features. Thus, adverse affects of reduced spacing between interconnect conductive features may be mitigated by interposing material having a low dielectric constant (low-.kappa. material) between the conductive features.
Typical dielectric material used in the manufacture of integrated circuits such as silicon oxide (SiO.sub.x) has a dielectric constant of about 4. However, recently, new materials having a lower dielectric constant (e.g., dielectric constants less than about 2.5) have been formed of porous silica. Generally, these porous silica films are formed by encapsulating a sacrificial material such as a polynorbornene within a silica matrix and heating the silica and sacrificial material to decompose the sacrificial material. Upon decomposition, the sacrificial material evaporates and permeates from the silica lattice, leaving air-filled voids within the lattice.
Generally, the dielectric constant of a two-phase film varies in accordance with the equation presented below: EQU ln(k.sub.c)=v.sub.1 ln(k.sub.1)+v.sub.2 ln(k.sub.2)
where k.sub.1, k.sub.2, and k.sub.c are the dielectric constant of phase 1, phase 2, and the two phase material, respectively, and v.sub.1 and v.sub.2 are the volume fraction of phase 1 and phase 2 in the material. Compared to dielectric film such as silica, air has a relatively low dielectric constant of 1. Thus, a dielectric constant of a two-phase material including air and silica may be reduced by incorporating more air into the two-phase material. However, as; more air is introduced into the material, the silica-based dielectric film tends to weaken and may be susceptible to damage during circuit use or subsequent circuit manufacturing processes such as chemical mechanical polishing. Accordingly, improved dielectric films having a low dielectric constant, which are suitable for microelectronic applications, are desirable.